Method for controlling sample introduction in microcolumn separation techniques and sampling device

ABSTRACT

In a method for controlling sample introduction in microcolumn separation techniques, more particularly in capillary electrophoresis (CE), where a sample is injected as a sample plug into a sampling device which comprises at least a channel for the electrolyte buffer and a supply and drain channel for the sample. The supply and drain channels discharge into the electolyte channel at respective supply and drain ports. The distance between the supply port and the drain port geometrically defines a sample volume. The injection of the sample plug into the electrolyte channel is accomplished electrokinetically by applying an electric field across the supply and drain channels for a time at least long enough that the sample component having the lowest electrophoretic mobility is contained within the geometrically defined volume. The supply and drain channels each are inclined to the electrolyte channel. Means are provided for electrokinetically injecting the sample into the sample volume. The resistance to flow of the source and drain channels with respect to the electrolyte buffer is at least about 5% lower than the respective resistance to flow of the electrolyte channel.

[0001] The present invention concerns a method for controlling sampleintroduction in microcolumn separation techniques. The invention alsoconcerns a respective sampling device for a controlled sampleintroduction in microcolumn separation techniques.

BACKGROUND OF THE INVENTION

[0002] Microcolumn separation techniques, in particular capillaryelectrophoresis has become a very interesting separation technique whichis used as part of a sensor or a chemical analysis system. One majorreason for this is the great efficiency of the method as a separationtechnique. The sampling methods usually applied in capillaryelectrophoresis are:

[0003] injection of a sample with a syringe, via a septum, in aninjection block,

[0004] the use of injection valves with/without a sample loop, and

[0005] dipping one end of the capillary tube into the sample reservoir,whereby the sample is introduced by gravity flow, by over- orunderpressure, or by electroendosmosis and/or electromigration.

[0006] While it is mentioned in Journal of Chromatography, 452, (1988)612-622, that sample valves are the most suitable sampling method forcapillary electrophoresis, there also is described a valveless devicefor the injection of a sample. The described arrangement comprises acast capillary block which is connected between an electrode compartmentand a sampling device. In the electrode compartment electrolytesolutions contact electrodes. The capillary tube contains measuringelectrodes which are connected with an evaluation electronics. Thesampling device consists of a broadened part of the capillary tubeconnected with two feeders which extend perpendicular to the capillarytube. The arrangement of the two feeders off-set from each other alongthe longitudinal extension of the capillary tube is such, that thesampling device has the shape of a capillary double T structure.

[0007] The sample is introduced into the sampling device via a syringe.The injection volume is defined geometrically by the distance which thetwo feeders are spaced apart along the capillary tube. The transport ofthe electolyte solution and the sample in the capillary tube isaccomplished by electric fields that are applied between the respectiveelectrodes along the capillary tube. An advantage of the double T shapesampling device, as is also obtained with the use of injection valves,is the concentration effect of dilute sample ionic species. However, itis possible that, allthough no electric field gradient over the feedersexists, sample components from the feeders may diffuse into thecapillary tube when the sample has already left the sampling position.The amounts of sample components that uncontrollably enter the capillarytube depend on the diffusion coefficients and the mobilities of therespective sample components. Thus, at the detector there not onlyarrives a more or less broadened plug of injected sample fluid,depending on the diffusion coefficients and the mobilities of therespective components in the electrolyte and the electric field, butalso the electrolyte in front and after or between individual plugs ofsample fluid is “polluted” with unpredictable amounts of samplecomponents. These unpredictable amounts of sample components reachingthe detector are highly undesirable and result in a high noise of thedetected signal, thus reducing the limits of detection considerably.

[0008] In Analytical Chemistry, 1992, 64, pages 1926-1932 a capillaryelectrophoretic device is described in which the sample is injectedelectrokinetically dipping one end of a capillary into the samplereservoir and applying a voltage across the ends of the capillary. Inthe electric field the sample is transported electrokinetically and isinjected at a T-junction into the channel system of the capillaryelectrophoretic device. This method, however, leads to a well-known biasof the actual sample composition due to the differences in theelectrophoretic mobilities of the sample components. Thus, the sampleintroduced often does not have the same composition as the originalsample. In addition, the volume of the introduced sample is very oftenunknown such, that internal standards have to be used for quantitativeanalyses.

SUMMARY OF THE INVENTION

[0009] It is therefore an object of the present invention to provide amethod for controlling sample introduction in microcolumn separationtechniques, and more particularly in capillary electrophoresis (CE), anda sampling device which overcomes the aforementioned disadvantages ofthe prior art. The sample volume shall be geometrically defined. Thecomposition of the sample which is injected shall not differ from theoriginal composition of the sample in the reservoir. The uncontrolledintroduction of sample fluid into the capillary tube shall be reducedconsiderably. If the unwanted leakage of sample fluid into the capillarytube cannot be totally avoided, provisions shall be made that at leastit only occurs in a predictable and controllable manner.

[0010] The method and the sampling device according to the inventionshall also allow an easy realization of miniaturized analysis concepts,such as the ones described, for example, in Sensors and Actuators B, 10(1993) 107-116. There the concept of a multi-manifold flow systemintegrated on a silicon substrate, with valveless switching of solventflow between channels and electro-kinetic pumping of an aqueous solvent,is described. A similar concept is described, for example, in AnalyticalChemistry, Vol. 64, No. 17, Sep. 1, 1992, 1926-1932. The describedminiaturized chemical analysis system on the basis of capillaryelectrophoresis comprises a complex manifold of capillary channels,which are micromachined in a planar glass substrate. The transport ofthe solvent and the sample occurs due to electro-kinetic effects(electro-osmosis and/or electrophoresis).

[0011] In order to meet all these and still further objects according tothe invention a method for controlling sample introduction inmicrocolumn separation techniques, in especially in capillaryelectrophoresis (CE) is provided, wherein an electrolyte buffer and amore or less concentrated sample are transported through a system ofcapillary channels. The sample is injected as a sample plug into asampling device which comprises at least a channel for the electrolytebuffer and a supply and drain channel for the sample. The channel forthe electrolyte buffer and the supply and drain channels for the sampleintersect each other. The supply channel and the drain channel for thesample, each discharge into the channel at respective supply and drainports. The distance between the supply port and the drain portgeometrically defines a sample volume. The supply and the drain channelseach are inclined to the electrolyte channel. The injection of thesample plug into the electrolyte channel is accomplishedelectro-kinetically by applying an electric field across the supply anddrain channels for a time at least long enough that the sample componenthaving the lowest electrophoretic mobility is contained within thegeometrically defined volume. By this measure the composition of theinjected sample plug will reflect the actual sample composition.

[0012] In a further preferred process step, immediately after theinjection of the sample plug, the electrolyte buffer is allowed toadvance into the supply channel and into the drain channel at therespective supply and drain ports for a time period, which amounts to atleast the migration time of a slowest component within the sample plugfrom the supply port to the detector. Thus, the sample is pushed backinto the respective supply and drain channels and substantiallyprevented from uncontrollably diffusing into the electrolyte bufferwhich is transported past the supply and drain ports. In addition themethod allows to control the sample composition within the electrolytebuffer.

[0013] The sampling device according to the invention comprises anelectrolyte channel, and a supply channel and a drain channel for thesample, which discharge into the electrolyte channel at respectivesupply and drain ports. The ports are arranged with respect to eachother such, that a sample volume is geometrically defined. The supplyand drain channels each are inclined to the electrolyte channel. Meansare provided for electro-kinetically injecting a sample into the samplevolume. The resistance to flow of the source and drain channels withrespect to the electrolyte buffer is at least about 5% lower than therespective resistance to flow of the electrolyte channel. Preferrendvariants of the method according to the invention and preferredembodiments of the sampling device according to the invention aresubject of the respective dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will become apparent from the following descriptionwith reference to the schematic drawings in which:

[0015]FIG. 1 is a schematic view of a microcolumn separation devicewhich comprises a sampling device according to the present invention,

[0016]FIG. 2 is a sectional view of the microcolumn separation deviceaccording to FIG. 1,

[0017]FIG. 3 is an enlarged view of the encircled part of themicrocolumn separation device according to FIG. 1, showing a firstembodiment of a sampling device, and

[0018]FIG. 4 is a second embodiment of the sampling device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0019] In FIGS. 1 and 2 an exemplary embodiment of a microcolumnseparation device, more particularly of an electrophoretic separationdevice, is depicted. It comprises a base part 1 and a lid part 2. Thebase part 1 can be made of glass, monocrystalin silicon or othermaterials known from semiconductor manufacture, or of a suitable polymermaterial. The lid part 2 is preferably made of glass. The base part 1comprises a channel system 4 which is etched, micromachined or otherwiseestablished in its surface. Preferably techniques known fromsemiconductor manufacture are applied for creating the channel system inthe surface of the base part 1. The lid part is provided with throughholes R, S, D, W, which communicate with the channel system 4 and areadapted to accomodate and hold the ends of capillary tubes. The lid part2 is also provided with various ports for light waveguides, which arepart of an optical detection system, such as, for example, afluorescence detection system, or an absorption detection system, or asystem for the detection of changes of the refractive index of a sampleflowing through the channel system. The ports are distributed along thechannel system 4 after a sampling device 3, where a sample is introducedinto an electrolyte buffer, thus allowing measurements at differentlocations along the channel system.

[0020] The transport of the electrolyte buffer and of the more or lessconcentrated sample is preferably accomplished by means of electricfields, which are created by switching electric potentials betweenelectrodes of a respective reservoir R and waste receptacles W for theelectrolyte buffer and between electrodes associated with respectivesource S and drain receptacles D for the sample.

[0021] In FIGS. 3 and 4 the encircled sampling device 3 of FIG. 1 isshown in an enlarged scale. It is part of the flow injection analysissystem of FIG. 1, which is based on electro-kinetic principles andallows an electrophoretic analysis of a sample. The sampling device 3 isan integrated part of the capillary channel system 4 and is thusconnected with the reservoir R and the waste receptacle W behind thedetectors 5-8, for the electrolyte buffer, and with the sourcereceptacle S and the drain receptacle D for the sample which is to beanalyzed. In FIGS. 3 and 4, for the sake of clarity the reservoir R andthe receptacles W, S, D are not drawn, but they are only symbolized byarrows, which at the same time indicate the direction of fluid flow inthe channel system 4.

[0022] In FIG. 3 a first exemplary embodiment of the sampling device isshown. It comprises a capillary channel piece 22, which on one end isconnected to a capillary channel comunicating with the reservoir R forthe electrolyte buffer and in longitudinal direction on the other endwith a capillary channel where the electrophoretic separation of thesample takes place, and which leads to the detector(s) and in furtherconsequence to the waste receptacle(s) W. The sampling device furthercomprises a supply channel 23, which communicates with a sourcereceptacle S for the sample, and a drain channel 24 which leads to adrain receptacle D. The source channel 23 and the drain channel 24 areinclined to the longitudinal extension of the channel piece 2,preferably they are arranged about perpendicular such, that togetherwith the channel piece 22 they form a double T structure, as shown inthe drawing. The source channel S and the drain channel D each dischargeinto the channel piece 22 at respective supply and drain ports 25, 26.According to the drawing in FIG. 3 the supply port 25 and the drain port26 are spaced apart from each other longitudinally at the channel piece22 such, that a sample volume 27 is geometrically defined as will beexplained in more detail hereinafter. It is to be understood, that thedrain channel 24 can be arranged in direct longitudinal extension of thesource channel 23 such, that the supply and drain ports 25, 26 aresituated opposite each other. In that case the channels of the samplingdevice have no double T structure, but they are arranged in form of anordinary crossing.

[0023] As already mentioned before the transport of the fluids, i.e. theelectrolyte buffer and the sample, is accomplished with electric fields,which are a result of different electric potentials at the reservoir Rand the waste receptacle W for the electrolyte buffer, and therespective source receptacle S and the drain receptacle D for thesample. By applying, for example, a positive electric potential to thereservoir R and a negative electric potential to the waste receptacle,the electrolyte buffer is electro-kinetically transported from thereservoir R through the capillary channel system to the waste receptacleW. In order to introduce the sample into the channel piece 22, forexample, the source receptacle S for the sample is maintained at apositive potential and the drain receptacle D is kept on a negativepotential. In the resulting electric field the sample is transportedelectro-kinetically from the source receptacle S to the drain receptacleD. The direction of flow is indicated in FIG. 3 by the arrows S, V, andD. By this measure, a part 27 of the channel piece 22, which isdelimited by the supply port 25 on the one end and by the drain port 26on the other end is filled with sample. Thus, the sample-filled part 27of the channel piece of the sampling device 3 defines the volume of theelectro-kinetically injected sample plug, which is indicated by thehatchings in FIG. 3. In other words, the volume 27 of the sample plug isgeometrically delimited by the spaced apart supply and drain ports 25and 26. In the aforemantioned case that the supply and drain ports arearranged opposite each other, such that the channel piece 22 and thesupply and drain channels 23, 24 form an ordinary crossing, the size andvolume of the intersection determines the sample volume. Thus, in thatcase, the sample volume is only defined by the cross-sections of therespective channels 22, 23, 24.

[0024] In order to assure that the composition of the sample in thesample volume 27 reflects the actual sample composition in the reservoirR the electric field across the supply and drain channels 23, 24 must bemaintained for at least for a time period long enough that thegeometrically defined sample volume is filled and contains the componentof the sample which has the lowest electrophoretic mobility. Thisminimum time period t_(min) is given by the equation t_(min)=d/μ_(i)−E.In this equation d stands for the distance, which the source and drainport are spaced apart; μ_(i) is the total mobility of the slowestcomponent i of the sample, which will be referred to in more detailhereinafter; E is the field strength across the source and drainchannels, which results from the difference in potentials.

[0025] When a electrophoretic analysis of a sample is to be carried out,first an electric field between the reservoir R and the waste receptacleW is established such, that the electrolyte buffer is transported fromthe reservoir R to the waste receptacle W. After the channel system ofthe chemical analysis system has been filled with the electrolytebuffer, the injection of the sample into the channel piece 22 isinitiated. For that purpose an electric field is established between thesource receptacle S and the drain receptacle D such, that the sample iselectro-kinetically transported from the source receptacle S through thesupply channel 23 via the channel piece 22 into the drain channel 24 andon to the drain receptacle D. It is understood that during the timeperiod, in which the sample is injected, the electric field between thereservoir R and the waste receptacle W is switched off, or that thepotentials are chosen such, that the sample only is transported alongthe path described above. After the injection time period which ischosen such, that it is ensured that the sample volume 27 between thesupply port 25 and the drain port 26 is filled with the sample, theelectric field between the source receptacle S and the drain receptacleD is switched off. At the same time the electric field between thereservoir R and the waste receptacle W is activated again such, that thesample contained within the sample volume 27 is transported on into thedirection of the detector(s) and the waste reservoir. While it istransported through the channel system the sample is separatedelectrophoretically in the electric field.

[0026] The problem of leakage or diffusion of sample components into theelectrolyte buffer while it is transported past the supply and drainports 23 and 24, even though no electric field is applied between thesource receptacle S and the drain receptacle D, is solved by allowingthe electrolyte buffer to advance into the supply channel 23 and intothe drain channel 24 at the respective supply and drain ports 25 and 26for a time period, which amounts to at least the migration time t_(i) ofthe slowest component i within the sample plug from the supply port 25to the respective detector. Thus, the sample is pushed back into thesupply and drain channels 23, 24 and substantially prevented fromuncontrollably diffusing into the electrolyte buffer which istransported past the supply and drain ports 25, 26.

[0027] The migration time t_(i) of the slowest component i of the sampleis defined as the quotient between the separation length L and theproduct of the total mobility μ_(i) of the slowest component i of thesample and the electric field strength E′ acting on it along its path L,and is given by the equation T_(i)=L/μ_(i)−E′). In this equation theseparation length L (FIG. 1) is the distance the sample component itravels between the supply port 25 and the respective activated detectorn8, and the total mobility μ_(i) of the component is the sum of theelectrophoretic mobility μ_(i,ep) of the component and the overallelectro-osmotic mobility μ_(eo) of the sample. The time period duringwhich the detection is accomplished is very short in comparison to themigration time of the slowest component of the sample and thus isneglectable.

[0028] In order to allow the electrolyte buffer to advance into thesupply and drain channels 23 and 24, in the exemplary embodiment of thesampling device depicted in FIG. 3 the source receptacle S and the drainreceptacle D are switched on an electric potential which is differentfrom the electric potential at the reservoir R for the electrolytebuffer, thus establishing a potential difference of suitable magnitude.In an embodiment, where the electrolyte buffer is transported from apositive potential to a negative potential, the potentials at the sourceand drain receptacles S, D are chosen negative with respect to thepositive potential at the reservoir R. In case of a transport of theelectrolyte buffer from a negative potential to a positive potential thepotentials of the source and drain receptacles S, D are chosen positivewith respect to the reservoir R.

[0029] Preferably the potential difference between the reservoir R andthe source and drain receptacles S, D is chosen such, that the resultantelectric field has a field strength which amounts to at least about 0.1V/cm.

[0030] Another approach to allow an advancement of the electrolytebuffer into the supply and drain channels 3, 4 is depicted in FIG. 4.The construction of the depicted sampling device 3 basically correspondsto the one depicted in FIG. 3. It comprises a channel piece 12 with twoside channels 13, 14. The side channels are inclined to the longitudinalextension of the channel piece 12 an angle that amounts to from about 5degrees to about 175 degrees; however, preferably they are arrangedabout perpendicular with respect to the channel piece 12. The sidechannels are a supply channel 13 and a drain channel 14, which dischargeinto the channel piece 12 at respective supply and drain ports 15, 16.Preferably the supply port 15 and the drain port 16 are spaced apartfrom each other at the channel piece 12 and delimit a sample volume 17.The distance d which they are spaced apart from each other typicallyamounts to from about 0 μm to about 3 cm, most preferably to about 3 mm,wherein the value 0 indicates that the supply and drain ports arelocated opposite each other. The channel piece 12 communicates with areservoir R and a waste receptacle W for the electrolyte buffer. Thesupply channel 13 is connected with a source receptacle S for thesample, while the drain channel 14 communicates with a drain receptacleD.

[0031] The sampling device 3 is part of an electrophoretic chemicalanlysis system and basically functions in the same way as the samplingdevice depicted in FIG. 3. However, in order to allow the electrolytebuffer to advance into the supply and drain channels 13, 14 theresistance of flow within the two channels is reduced. In particular thesource channel and the drain channel each have a resistance to flow withrespect to said electrolyte buffer, which is about 5% lower than therespective resistance to flow of said electrolyte channel. Surprisinglythe reduction of the resistance to flow of the supply and drain channels13, 14 does not result in an increase of the leakage or diffusion ofsample components into the electrolyte buffer as it is transported pastthe respective supply and drain port 15, 16. Instead, the reduction ofthe resistance to flow of the side channels 13, 14 leads to a convectiveflow of the electrolyte buffer into the channels 13, 14, even when theaplied electric fields should not result in such a flow. Thus, theleakage or diffusion of sample components is considerably decreased andthe noise of the detected signal is reduced. In consequence thesensitivity of the analytic system, that is the limit of detection, isincreased. The resistance to flow of the supply and drain channel can bedeminished by either reducing the length of the respective channels orby increasing their respective widths w. Preferably the reduction of theresistance to flow of the supply and drain channels 13, 14 is achievedby providing them each with a width w that is at least about two timesgreater than the width p of the supply and drain ports 15, 16. Such, thesupply and drain channels 13, 14 each have about the shape of a bottle,the bottle neck being the respective supply or drain port 15, 16.

[0032] While it is possible that the supply and drain channels 13, 14empty directly into the channel piece 12 such, that their ends, whichare located night next to the channels piece 12 are the respectivesource and drain ports 15, 16, from where the width of the channelsgradually increases over a respective intermediate piece 13′, 14′ fromthe width p of the ports to the final width w of the channels, thesupply and drain ports also have longitudinal extensions 1. Theslongitudinal extensions correspond at least to the width p of therespective supply and drain ports 15, 16. It is advantageous, if thewidths p of the supply and drain port 15, 16 are kept constant alongtheir longitudinal extension 1. In a preferred embodiment the widths pof the supply and drain port 15, 16 are chosen such, that they aboutcorrespond to the width b of the channel piece 12.

[0033] The depth of the channel piece 12 (which should correspond to thedepth of the channel system that it is part of) and of the supply anddrain channels 13, 14 typically amounts to from about 0.1 μm to about100 μm. The depths of the bottle-neck-like supply and drain port 15, 16about corresponds to the depth of the channels.

[0034] The sampling device 3 according to the invention has beenexplained with reference to exemplary embodiments which are part ofmicro-analysis chips. It can as well be an arrangement of capillarytubes, which is part of a electrophoretic chemical analysis system madeof capillary tubes. In the most preferred embodiment, however, thesampling device is integrated into a system of capillary channels whichare established in a small planar sheet of glass, semiconductormaterial, or a suitable polymer. Advantageously the channel systemincluding the supply and drain channels and the respective supply anddrain ports are etched or micromachined or casted (in case of a polymerbase part), or otherwise established in the planar substrate. Mostsuitable for its manufacture are techniques which are well establishedin semiconductor production or in the manufacture of micromechanicalelements.

[0035] The combination of a structure that geometrically defines theinjected sample volume with an electro-kinetic injection of the sampleover a defined minimum time period allows to relyably control the samplevolume and to assure that the composition of the sample contained withinthe sample volume reflects the original composition of the sample in thereservoir. A further improvement of the method and the sampling deviceaccording to the invention allows a considerable reduction ofuncontrolled leakage or diffusion of sample components into theelectrolyte buffer. Thus, it is possible to reduce the leakage ordiffusion such, that the still occuring leakage results in aconcentration of the sample in the electrolyte buffer, that is less than3% of the original concentration of the sample. By this measure thenoise of the detected electrophoretic signal is reduced and thedetection limits are increased.

What is claimed is:
 1. A method for controlling sample introduction inmicrocolumn separation techniques, in especially in capillaryelectrophoresis (CE), wherein an electrolyte buffer and a more or lessconcentrated sample are transported through a system of capillarychannels, and wherein said sample consisting of components havingdifferent electrophoretic mobilities is injected as a sample plug into asampling device, which comprises at least a channel for the electrolytebuffer and a supply and drain channel for said sample, said electrolytechannel and said supply and drain channels for said sample intersectingeach other, and said supply and drain channels each being inclined tosaid electrolyte channel and discharging into it at respective supplyand drain ports, wherein a distance between said supply port and saiddrain port geometrically defining a sample volume, characterized in thatthe injection of said sample plug into said electrolyte channel isaccomplished electrokinetically by applying an electric field acrosssaid supply and drain channels for a time period which is at least longenough, that a slowest of said sample components having the lowestelectrophoretic mobility is contained within said geometrically definedvolume.
 2. A method according to claim 1 , wherein said time periodcorresponds at least to the time defined by the equationt_(min)=d/(μ_(i)−E), wherein d stands for the distance between saidsupply and drain ports, μ_(i) is the total electro-kinetic mobility ofsaid slowest component, and E stands for the electric field strengthacross said source and drain channels.
 3. A method according to claim 2, wherein said electrolyte buffer and said sample are transportedelectrokinetically, and further wherein immediately after said injectionof said sample plug said electrolyte buffer is allowed to advance intosaid supply channel and into said drain channel at said respectivesupply and drain ports for a time period, which amounts to at least amigration time of a slowest component within said sample plug from thesupply port to a detector, thus pushing back said sample into saidrespective supply and drain channels and substantially preventing saidsample from uncontrollably diffusing into said electrolyte buffer whichis transported past said supply and drain ports.
 4. A method accordingto claim 3 , wherein within said time period said sample in said supplyand drain channels is subjected to an electric potential, which isdifferent from an electric potential at a reservoir for said electrolytebuffer, thus establishing a potential difference such, that saidelectrolyte buffer is allowed to advance into said supply channel andinto said drain channel.
 5. A method according to claim 4 , wherein saidpotential difference is chosen such, that a resultant electric fieldstrength amounts to at least about 0.1 V/cm.
 6. A method according toclaim 3 , wherein said electrolyte buffer is-allowed to advance intosaid supply and drain channels by reducing a resistance to flow withinsaid supply and drain channels.
 7. A method according to claim 6 ,wherein said resistance to flow is reduced by either reducing thelengths of said supply and drain channels or by increasing theirrespective widths.
 8. A method according to claim 7 , wherein saidresistance to flow of said supply and drain channels is reduced byproviding said supply and drain channels each with a width that is atleast about two times greater than a width of said supply and drainports.
 9. A method according to claim 8 , wherein said supply and drainports of said respective supply and drain channels have a longitudinalextension which corresponds at least to about said widths of said supplyand drain ports, and that said widths are kept about constant along theextension of said supply and drain ports.
 10. A method according toclaim 9 , wherein said widths of said supply and drain ports are chosensuch, that they about correspond to a width of said channel piece.
 11. Amethod according to any one of the preceeding claims, wherein saidsystem of capillary channels including said supply and drain channelswith their respective ports are etched or micromachined or otherwiseestablished in a planar substrate made of glass, or semiconductormaterials, or a suitable polymer, or the like.
 12. A sampling devicecomprising a channel for an electrolyte buffer and a supply channel anda drain channel for a sample, which each discharges into saidelectrolyte channel at a respective supply and a drain port, which portsare located with respect to each other such, that a sample volume isgeometrically defined, said supply and drain channels each beinginclined to a longitudinal extension of said channel piece, and meansfor electrokinetically injecting a sample into said sample volume,characterized in that said source channel and said drain channel, eachhave a resistance to flow with respect to said electrolyte buffer, whichis about 5% lower than the respective resistance to flow of saidelectrolyte channel.
 13. A sampling device according to claim 12 ,wherein said supply and said drain channel each has a width that is atleast about two times greater than a width of said supply and drainports.
 14. A sampling device according to claim 13 , wherein said supplyand drain ports of said respective supply and drain channels have alongitudinal extension which corresponds at least to about said widthsof said supply and drain ports, and that said widths are about constantalong the extension of said supply and drain ports.
 15. A samplingdevice according to claim 14 , wherein said widths of said supply anddrain ports about correspond to a width of said channel piece.
 16. Asampling device according to claim 15 , wherein said channels have adepth of from about 0.1 μm to about 100 μm.
 17. A sampling deviceaccording to claim 12 , wherein said supply port and said drain port arespaced apart from each other a distance which amounts to from about 0 μmto about 3 cm, preferably to about 3 mm.
 18. A sampling device accordingto claim 12 , wherein said supply channel and said drain channel,respectively, each are inclined with respect to said longitudinalextension of said channel piece an angle that amounts to from about 5degrees to about 175, preferably to about 90 degrees and further whereinsaid supply channel and said drain channel extend about parallel to eachother.